update: replace workingctx.dirty and raising Abort by cmdutil.bailifchanged
This patch makes wrapping "commands.update()" by largefiles extension
useless, because "cmdutil.bailifchanged()" can detect changes of
largefiles in the working directory.
This patch also changes test-update-branches.t, because
"cmdutil.bailifchanged()" shows more detailed information about
dirty-ness of the working directory than "workingctx.dirty()".